The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the in vivo imaging of metabolites using paramagnetic labels such as hyperpolarized carbon-13.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The advent of hyperpolarized (carbon-13) has spurred interest in imaging with this isotope in a variety of in vivo applications such as vascular imaging (MRA) and metabolic flux. One of these is C-13 labeled pyruvate and its metabolites (lactate and alanine), which are of particular interest in oncology applications. The NMR spectrum of these three metabolites is relatively sparse, making them well suited for chemical shift based imaging methods such as those commonly used to separately image water and fat. As shown in FIG. 2, these metabolites have a single peak for lactate (Lac), a single peak for alanine (AL) and two peaks for pyruvate (PE and Pyr). In a 3 Tesla polarizing field the chemical frequency shift of alanine is approximately −242 Hz relative to lactate, pyruvate has a main peak at approximately −622 Hz and a pyruvate ester peak lies at approximately −242 Hz relative to lactate.
The usual method for imaging these C-13 isotopes uses an echo planar spectroscopic imaging technique that requires the acquisition of large amounts of data. In order to resolve the spectral peaks for AL and PE it is necessary to acquire 64 NMR signal at different echo times. A large amount of data is needed to obtain the needed “spectral resolution” when a Fourier transformation is performed on the data to produce a spectrum. Because of the large amount of data that is acquired, a difficult choice between shorter scan time and higher spatial resolution is usually required.
Recently, a new method known as IDEAL was developed for imaging spin species such as fat and water. As described in U.S. Pat. No. 6,856,134 B1 issued on Feb. 15, 2005 and entitled “Magnetic Resonance Imaging With Fat-Water Signal Separation”, the IDEAL method employs pulse sequences to acquire multiple images at different echo times (TE) and an iterative, linear least squares approach to estimate the separate water and fat signal components. The advantage of the IDEAL method is if the frequencies of the particular metabolites being imaged are known, the number of different echo time repetitions can be significantly reduced. This “a priori” information shortens scan time and enables more pulse sequence repetitions to be devoted to increased image resolution.